Electronic devices that perform wireless communications are each provided with an antenna compliant with a communication method, and various antennas have been developed to meet diversification of the communication method.
For example, techniques related to Radio Frequency Identification (RFID), in which various kinds of information about an article is recorded in an IC chip and read out by radio, have been developed to realize article management such as inventory management, merchandise management and distribution management set as a target. In an RFID system, an RFID tag attached to an object has an antenna and a memory where information about the object is accumulated. A reader/writer connected to a host computer reads the information from the RFID tag and transmits the read information to the host computer by using radio signals. Conversely, the reader/writer writes information transmitted from the host computer into the memory of the RFID tag by using radio signals. Traceability of the object is improved by the RFID system. Moreover, work labor is reduced in comparison with conventional management that employs display of letters and signs.
An article to which an RFID tag is attached is desired to be as small as possible or to be thin and thus, an antenna used for the RFID tag generally includes a strip line. The antenna including the strip line is called a microstrip antenna. Main antennas include a dipole antenna, a folded dipole antenna, a patch antenna and a bowtie antenna.
FIG. 1 is an external view of an example of the structure of a microstrip antenna based on a conventional technique, and FIGS. 2A and 2B are diagrams for describing an operation of the antenna illustrated in FIG. 1. FIG. 2A schematically illustrates the antenna together with an electric field, and FIG. 2B illustrates an electric field intensity in a graph. Further, FIGS. 3A and 3B are diagrams that illustrate an electric field distribution and a magnetic field distribution, respectively, of the antenna illustrated in FIG. 1. FIG. 3A illustrates an electric field distribution and FIG. 3B illustrates a magnetic field distribution, when the antenna is viewed in a direction perpendicular to an antenna element.
An antenna A1 illustrated in FIG. 1 is a rectangular microstrip antenna and has a ground 3, a dielectric substrate 4 and an antenna element 1. A feeder line 5 is connected to a feeding point 2 of the antenna element 1, and the feeder line 5 penetrates the dielectric substrate 4. A high frequency signal is supplied from the feeding point 2 to the antenna element 1 through the feeder line 5 and emitted as a radio wave. At this time, as illustrated in FIG. 2A, an electric field is produced along arrows 7 and between the antenna element 1 and a map 9 of an antenna element virtually assumed within the ground 3. Also, at a certain moment, an electric field is produced as indicated by arrows 10 in FIG. 2B between the antenna element 1 and the map 9, and the electric field has an intensity as indicated by the graph in FIG. 2B. Furthermore, around the antenna element 1, an electric field is produced along arrows 11 illustrated in FIG. 3A, and a magnetic field is produced along arrows 12 illustrated in FIG. 3B.
RFID tags are attached to many kinds of object, and the object may be made of metal, organism or liquid that readily affect radio waves. Therefore, the antenna of the RFID tag is desired: (1) to be small and (2) to be capable of sufficiently maintaining a property even when being placed near metal, organism or liquid. In particular, as to the desire (2), ideally, the antenna of the RFID tag that receives radio waves from a reader/writer has a property that does not depend on an environment in which the RFID tag is attached. Actually however, in a state in which metal is brought close to the RFID tag or when the RFID tag is attached to the metal, the property of the antenna greatly deteriorates so that a distance that enables communications with the reader/writer may not be maintained.
In view of such a situation, a RFID tag called a cloth tag (for example, see http://www2.nict.go.jp/pub/whatsnew/press/h17/060224/060224.html) and a RFID tag called a KU-Tag (for example, see http://www.ittc.ku.edu/˜deavours/kutag.html, http://www1.rfidjournal.com/article/articleprint/2275/-1/1/, http://www.ittc.ku.edu/publications/KU-Tag_flyer.pdf, and http://www.ittc.ku.edu/˜deavours/pubs/ccct06.pdf) have been proposed as applications of the microstrip antenna illustrated in FIG. 1. These RFID tags are configured not to be affected by the material of an attachment surface, by a principle of using a ground surface as the attachment surface.
FIG. 4 and FIG. 5 are external views of RFID tags based on conventional techniques. FIG. 4 illustrates the RFID tag having a ground surface as an attachment surface, and FIG. 5 illustrates an example of the KU-Tag in which a semiconductor chip is connected to two points of an antenna element.
An RFID tag T2 illustrated in FIG. 4 includes a rectangular microstrip antenna, and specifically includes: an antenna element 24, a ground 26, a first dielectric substrate 25 disposed between the antenna element 24 and the ground 26, and a second dielectric substrate 27 to be interposed between an object to which the RFID tag T2 is to be attached and the ground 26. The RFID tag T2 further includes a semiconductor chip 22 electrically connected to a feeding point 21 on the antenna element 24 and to a feeding point 23 on the ground 26 and wirelessly communicating via the antenna. The semiconductor chip 22 supplies signals to the antenna element 24 and the ground 26 (unbalanced power supply). In the RFID tag T2, of the antenna element 24 and the ground 26, the ground 26 is attached to the object so that influence of the material of the object is suppressed.
An RFID tag T3 illustrated in FIG. 5 includes: an antenna element 31, a ground 33, a first dielectric substrate 32 interposed between the antenna element 31 and the ground 33, a second dielectric substrate 35 to be interposed between an object to which the RFID tag T3 is to be attached and the ground 33, and a semiconductor chip 29, which are similar to the RFID tag T2 in FIG. 4. However, in the RFID tag T3 illustrated in FIG. 5, the semiconductor chip 29 is electrically connected to two points at one side of the antenna element 31 in a rectangular shape, which is different from the RFID tag T2 in FIG. 4. A signal is supplied from the semiconductor chip 29 to the antenna element 31 via feed patterns 28 and 30 (balanced power supply). In the RFID tag T3 as well, the ground 33 is attached to an object so that influence of the material of the object is suppressed.
Also, as an antenna that suppresses influence of the material of an object, there have been proposed antennas that each function as a slot antenna in a state of being brought close to a metal face and function as a normal antenna when being moved away from the metal face (for example, see U.S. Pat. No. 6,914,562 and No. 6,501,435)
However, each of the RFID tags illustrated in FIG. 4 and FIG. 5 may not be always attached to the metal face, and when, for example, the RFID tag is attached to a material other than the metal, the ground of the RFID tag is not in a state of a short with respect to a peripheral ground potential. In other words, impedance in the ground of the RFID tag may not always short with respect to the peripheral ground potential, and still depends on a condition of the material of the object. For this reason, in an attempt to enlarge the ground to the extent that the property of the antenna is not impaired, the size of the RFID tag is increased and falls outside a practically acceptable range. It is generally desirable in practical use that the size of the RFID tag be equal to or less than a business card, but it is difficult to realize this size with the RFID tags illustrated in FIG. 4 and FIG. 5.
Further, the antenna, which functions as the slot antenna when being brought close to the metal face and functions as the normal antenna when being moved away from the metal face, has a complicated structure, making it difficult to reduce the size. Moreover, this problem is not limited to the RFID tag and rather a common problem among electronic devices that each performs wireless communication through an antenna.